Proof of ion-pair structures in ammonium-based protic ionic liquids using combined NMR and DFT/PCM-based chemical shift calculations

Abstract

The self-assembly of triethylammonium bis(trifluoromethylsulfonyl)imide, i.e. [(C₂H₅)₃NH][TFSI], in chloroform and aqueous solutions has been investigated using ¹H NMR spectroscopy and computational (DFT/PCM prediction) methods. We have examined a number of ion pairs formed between the [(C₂H₅)₃NH]⁺ cation with different conformations of alkyl substituents as well as various dispositions of the multi-site [TFSI]⁻ anion. Based on the agreement between the calculated (DFT) and observed ¹H NMR chemical shifts, [(C₂H₅)₃NH][TFSI] in chloroform formed lipophilic complexes with effective N⁺–H⋯N or N⁺–H⋯O hydrogen bonding, whereas hydrophilic complexes with C^α–H⋯O and C^α–H⋯F hydrogen bonding are found in aqueous solutions. This study provides a new insight into the self-aggregation of ammonium PILs incorporating the widely used [TFSI]⁻ anion and demonstrates the importance of solvent effects on chemical shifts. The simulations with explicit and implicit dielectric continuum solvents are found to be the most realistic method, yielding a representative ensemble of structures.